New techniques, or combinations of techniques, will be investigated to protect the intestine from radiation damage during intensive radiotherapy to abdominal tumors. If this goal can be achieved, higher radiation doses and dose-rates might be given safely in efforts to control or to cure these tumors, without damaging the intestine or causing "radiation sickness" in the patient. Successful experimental and clinical radiation protection of the normal kidney utilizing selective vasoconstrictor infusions into the renal artery has already been attained, and recent preliminary experimental results with the selective infusion of Pitressin into the superior mesenteric artery and intravenous infusions of Vasopressin have shown the feasibility of protecting the digestive trace in dogs. The creation of temporary ischemia of the normal organ (kidney or intestine) appears to be responsible for the radioprotective effect of a vasoconstrictor infusion. Experimental studies of the efficacy of several vasoconstrictors given by different routes (intra-arterial and intravenous) are planned, including electromagnetic blood flow determinations correlated with selective angiographic studies. Particular attention will be focused initially upon further investigations of the use of Pitressin in radiation protection of the digestive tract.